1. Field of the Invention
The present invention relates generally to methods for the Non-Destructive Evaluation (NDE) of pipes and tubes using magnetostrictive sensor technologies. The present invention relates more specifically to an improved method for detecting defects by manipulating the frequency characteristics of the transmitted and/or received signals associated with the use of magnetostrictive sensor technologies for the inspection of pipes and tubes.
2. Description of the Related Art
Magnetostrictive sensor technologies have been used successfully for a period of time with the inspection of pipes and tubes in processing plants such as refineries, chemical plants, steam process plants and the like. Examples of the use of magnetostrictive sensors, and the various analytical techniques associated therewith, are disclosed in U.S. Pat. Nos. 5,456,113 and 5,457,994, each entitled Non-Destructive Evaluation of Steel Cables and Ropes Using Magnetostrictively Induced Ultrasonic Waves and Magnetostrictively Detected Acoustic Emissions, as well as U.S. Pat. No. 5,581,037 entitled Non-Destructive Evaluation of Pipes and Tubes Using Magnetostrictive Sensors, all of which are commonly owned by the assignee of the present invention, Southwest Research Institute.
The techniques associated with such NDE inspections of pipes, tubes, cables and the like typically involve generating longitudinal waves along the length of the pipe or tube and analyzing signals that are reflected from defects and anomalies within the pipe or tube. One of the many advantages of this technique is the ability to detect defects by sensing the reflected signal at the same physical location at which the interrogating signal waves are generated.
Because mechanical waves generated by magnetostrictive sensors can propagate a long distance through a structure under inspection the techniques are capable of inspecting very long or large segments, typically more than a hundred feet under favorable conditions, of pipe very rapidly. These techniques also provide a complete volumetric inspection of a long section of pipe with minimum ancillary activity such as surface preparation, scaffolding, or insulation removal. These magnetostrictive sensor methods therefore offer a very efficient and comprehensive mechanism for pipe and tube inspection.
In general, the longitudinal wave modes utilized in the above referenced techniques for inspection are dispersive in nature. This means that the velocity of the mechanical wave propagation varies with the wave frequency. Various factors affect the dispersion characteristics of a transmitted signal containing broad frequency components. Primary among these factors are pipe geometry (diameter and wall thickness) and the presence or absence of a fluid within the pipe. Properly understood, the dispersive nature of the wave modes utilized in pipe inspection can assist in the identification of relevant anomalies and geometric irregularities in the pipe.
Examples of previous attempts in the prior art to improve the defect detectability of NDE sensors include the following patents:
U.S. Pat. No. 5,612,495 issued to Shimada et al. on Mar. 18, 1997, entitled Non-Destructive Examination Device, describes a system that uses magnetostrictive transmitters and response sensors to carry out the non-destructive evaluation of a material. The system anticipates the use of a resonant frequency for the interrogating signal.
U.S. Pat. No. 5,526,689 issued to Coulter et al. on Jun. 18, 1996, entitled Acoustic Emission for Detection of Corrosion Under Insulation, describes a method and apparatus for detecting the presence of surface corrosion under insulation on a pipe structure. This patent anticipates the use of a broadband of acoustic waves to interrogate the structure. The signal analysis method in Coulter et al. involves producing RMS voltage signals indicative of the detected sound waves and comparing the RMS voltage signals to standard signals obtained from uncorroded piping. The analysis involves a strict amplitude comparison to distinguish the signal component from the defect.
U.S. Pat. No. 5,195,046 issued to Gerardi et al. on Mar. 16, 1993, entitled Method and Apparatus for Structural Integrity Monitoring, describes a piezoelectric transducer based system designed for the detection, monitoring, and analysis of such materials as aircraft structures. Various pattern recognition techniques are utilized. The patent lists 25 illustrative features (column 11) that include both time and frequency domain parameters as providing the basis for pattern recognition.
U.S. Pat. No. 5,665,913 issued to Chung on Sep. 9, 1997, entitled Method and Apparatus for Evaluation and Inspection of Composite-Repaired Structures, describes a system and method for NDE of composite-repaired structures wherein the signal transmitters and sensors are piezoelectric based devices. The system anticipates the use of either a single frequency for interrogating the material or a range of frequencies.
U.S. Pat. No. 5,469,060 issued to Meyerand on Nov. 21, 1995 entitled Time Encoded Magnetic Resonance Imaging, describes a system that utilizes a separate set of signal transducers and applies a resonant frequency pulse to the material under investigation. The RF signals received as a function of time are converted to a set of frequency domain functions at specific times relating to specific strips in the image being generated.
U.S. Pat. No. 5,574,639 issued to Qian et al. on Nov. 12, 1996, entitled System and Method for Constructing Filters for Detecting Signals Whose Frequency Content Varies With Time, is generally directed to a signal analysis method intended to reliably detect the presence of signals of interest, especially those whose frequency content varies with time. The Qian et al. invention is directed to the development and use of time templates designed to match the received signal of interest. The patent applies the Gabor Spectrogram to a computer representation of the signals in the joint time-frequency domain for the purpose for instantaneous frequency estimation.
U.S. Pat. No. 5,144,839 issued to Lochner on Sep. 8, 1992 entitled Method of Checking the Presence of a Pipeline Connection Between Two End Points, describes a method for detecting pipe junctures with an acoustic signal whose frequency has been selected to be above the frequency of ambient noises and below an upper limit related to the geometry of the pipe and the acoustic velocity of the material in the pipe.
U.S. Pat. No. 4,937,767 issued to Reuschel et al. on Jun. 26, 1990 entitled Method and Apparatus for Adjusting the Intensity Profile of an Ultrasound Beam, describes an ultrasonic imaging system having adjustable signal sources such that the duration of the drive pulse can be varied. In the case of ultrasonic signal generators (piezoelectric crystals) , signal amplitude may be varied by changing the drive frequency around the crystal's resonant frequency.
U.S. Pat. No. 5,425,272 issued to Rhodes et al. on Jun. 20, 1995 entitled Relative Resonant Frequency Shifts to Detects Cracks, describes yet another ultrasonic measurement method that utilizes the ratio of two prominent resonant response frequencies to characterize the physical characteristics of a part.
U.S. Pat. No. 5,549,111 issued to Wight et al. on Aug. 27, 1996 entitled Method and Apparatus for Adjustable Frequency Scanning in Ultrasound Imaging, describes an ultrasonic system that utilizes an energy pulse having a carrier frequency higher in the center of the field of view than at the edges. A corresponding method for demodulation of the received signal is also described. In each case, it is a directional variable that is modified by means of the frequency variations.
U.S. Pat. No. 5,537,876 issued to Davidson et al. on Jul. 23, 1996 entitled Apparatus and Method for Non-Destructive Evaluation of Butt Welds, describes a system for interrogating welds in sheet material using horizontal shear ultrasonic waves generated on the surface. The apparatus described detects only reflected horizontal shear waves by electronically filtering out other modes that occur at different frequencies. The system utilizes electromagnetic acoustical transducers (EMAT) as both transmitters and receivers for the ultrasonic waves.
While utilizing a broadband interrogating signal with magnetostrictive sensor techniques generally facilitates the acquisition of a large quantity information by broadly "illuminating" the geometric and material features of the pipe or tube under investigation, this same quantity of information carried in the detected signal often becomes too complex to adequately analyze. It would be desirable, therefore, to gain the benefits of a broadband interrogating signal while still reducing the detected signal to an interpretable complexity. The present invention addresses one such signal complexity that can be appropriately eliminated or reduced to the point where otherwise obscured signal characteristics can be identified.